Structure and Reactivity of Sub-Monolayer Pt on CeO2 Surface from First Principles Thermodynamics

Transition metal catalysts supported on metal-oxide surfaces are promising materials for usage in heterogeneous catalysis due to strong metal-support interaction, which affects the intrinsic activity at the surface, and results in enhanced dispersion of metal atoms in some cases, thus promoting stability. One of the most widely investigated such systems are Pt/CeO₂ systems, which have been found to be highly active toward the water gas shift reaction [1]. Experimental and theoretical studies point to electron and oxygen transfer between adsorbed Pt atoms and CeO₂ support as the reason for the enhanced activity in such systems [2, 3]. The effect of this metal-support interaction, however, is far from clear in context of the redox reactions that occur in DMFCs and PEMFCs. Especially, the role of CeO₂, well known for its oxygen storage capacity, remains yet to be investigated in co-catalysis. To this end, understanding and characterizing the dispersion of Pt on the CeO₂ surface is of utmost importance. Stability and activity of a few Pt layers on rutile metal-oxides has been investigated computationally, yielding insights into the geometric and electronic influences of the support on the catalyst [4]. Sub-monolayer deposition of catalyst atoms opens up the support surface to direct engagement with the reactants and is well worth investigating. In this study, first principles DFT calculations are used in combination with rigorous statistical mechanics methods to find stable phases of Pt atoms dispersed on the CeO₂ (111) surface as a function of their chemical potential.  Employing the Cluster Expansion formalism in tandem with the Metropolis Monte Carlo Method to account for the configurational contribution to the system’s free energy [5], the important interactions dictating the surface morphology are identified. In agreement with experiments [2], a remarkable transfer of oxygen from CeO₂ to the Pt atoms is observed, and a tendency of Pt atoms to aggregate at low surface coverage is also seen. Further, using a linear Gibbs energy relation, the energetics of various reaction steps in a 2 e- oxygen reduction (ORR) process of the Pt/CeO₂ system are computed for several instances of oxygen co-adsorption and are compared to those of a model Pt (111) system as a function of the electrode potential. Additionally, the effect of support on the dissolution potential of Pt atoms is also evaluated to understand the stabilizing effect of the support, which has been observed experimentally [6]. Enhancement in ORR kinetics for Pt/CeO₂ systems has been observed experimentally, but such studies are few [7]. Two mechanisms are proposed in this study to explain such a behavior: (1) the preferential adsorption of OH on only Pt in contrast to the co-adsorption of O on both Pt and CeO₂, and (2) the activation of O-O bond breaking due to the oxygen starved CeO₂ surface. In principle, such systems with the possibility for the support material to take part in the reaction along with the catalyst atoms can open up new channels that facilitate ORR.

References:

[1] Bruix, A., et al. Journal of the American Chemical Society 134.21 (2012): 8968-8974.

[2] Vayssilov, G. N., et al. Nature materials 10.4 (2011): 310-315.

[3] Happel, M., et al. Journal of Catalysis 289 (2012): 118-126.

[4] Tripković, V., et al. ChemCatChem 4.2 (2012): 228-235.

[5] Han, B., et al. The Journal of Physical Chemistry C 116.10 (2012): 6174-6183.

[6] Trogadas, P., et al. Chemical Communications 47.41 (2011): 11549-11551.

[7] Lim, D., et al. Electrochemistry Communications 10.4 (2008): 592-596.